January 2012

Despite some of the teleconnections starting to lean towards a cooler eastern third of the country, the warm signals that remain are still more than enough to combat the cold to win out the month. Unlike January and February, the teleconnections are actually looking somewhat promising for a cold pattern in the eastern U.S. for a decent chunk of the month, which provides the eastern areas with an increased chance of snowfall. I believe that the Mid-Atlantic could see near normal monthly snowfall this month, but the risk is to the low side.

Here’s a look at the previous February forecast for comparison… not as much of a flip compared to what I had to do for December and January!

I drew up a basic primer on how to read a Skew-T sounding. Click here or on the image to expand the image.

There is so much that you can use a sounding for, and all of that information can be derived from the three basic plots: Temperature, dewpoint and wind. A great amount of the atmospheric details that you can derive from a sounding can be found in this PDF: http://www.met.tamu.edu/class/atmo251/Skew-T.pdf

Granted, it won’t be a lot of snow… mostly just a quick shower for the MD/DC region along a cold front that will push through the region tonight. To the north and west, the snow is expected to be a bit more robust as a quick thump of snow along a squall line of sorts (which could be thundersnow in PA as the lapse rates allow for conditional instability) could lead to accumulations of around one inch in the upslope areas of western/central PA down through the MD panhandle into east-central WV.

The strength of the snow squall varies on the models… here is a look at one of the more potent outcomes via the HRRR composite reflectivity around mid-evening:

And this is how it looks a couple hours later as it approaches the DC region… note that it has weakened a fair amount by this point:

East of the mountains could see a quick dusting to an inch of snow from this line of snow as it pushes through the region. This is much more likely in the northern Mid-Atlantic (PA down through around DC) than it will be in the southern areas as the upper-level energy driving this snow squall slides eastward across PA:

You don’t get a snow squall along a cold front in the Mid-Atlantic that often as temperatures are typically too warm ahead of the cold front, but this time the low-level temperatures should be cool enough to allow for snow, especially the further north you go.

Winds are starting to gust again today as another cold front makes its way through the region. A lack of moisture out ahead of the front means nothing but an isolated shower or two is possible along the front as is slides eastward.

Despite being a cold front, it won’t do much to bring in cooler air as temperatures drop into the upper 20s to lower 30s in the DC region tonight, with highs in the upper 40s to lower 50s tomorrow afternoon.

The synoptic setup is somewhat favorable for damaging winds and perhaps an isolated tornado or two tonight into early tomorrow in the southern Mid-Atlantic as storms ahead of a cold front push through the region. Poor lapse rates will likely prevent severe weather from occurring in DC, Baltimore and points north and west.

The storm system will be what we typically see for severe weather setups in the Mid-Atlantic during the winter and early spring… low CAPE/high shear. This system was looking fairly marginal for the region up to this morning due to the poor mid-level lapse rates, but it appears that the previously under-forecast dynamics will help introduce better lapse rates to the southern Mid-Atlantic. It is still fairly borderline, but with a strengthening low-level jet producing 50-60 kt. winds at 925mb it wouldn’t take much to bring those damaging winds to the surface.

925mb winds at 4am:

Forecast sounding for Richmond at 4am:

The sounding shows that there is enough directional shear for the anticipated storms to produce a tornado or two in the southern Mid-Atlantic tonight into tomorrow morning.

SPC’s outlooks for tonight into tomorrow morning:

I am mostly in agreement with them, but I would tone it down to a SEE TEXT instead of a Slight Risk north of the NC/VA border as the storms don’t look to be terribly organized, which is something you want to see for a Mid-Atlantic low CAPE/high shear system (at least for the damaging wind potential).

The SPC has released their first Moderate Risk for severe weather of the season for today:

The biggest risk will be for damaging winds along a squall line that is expected to form during the late evening to early morning hours just west and along the Mississippi River from southern Illinois southward into central Mississippi. 50-55 kt. winds in the 925-950mb level could mix down to the surface as the squall line pushes through. This is also a fairly good low CAPE/high shear setup for tornadoes, which could develop along the squall line.

The one thing to keep an eye on is the somewhat stable layer of air between the surface and 950mb, which will inhibit the potential severity for most of the storms except maybe in the southernmost areas.

I think the SPC prediction for today is fairly reasonable, but I will say that the northern extent of the Slight Risk and Moderate Risk could be cut back as the low-level air is forecast to be too stable north and east of southern Illinois. If the warm front associated with this system can get advected further north than forecast, then the SPC’s risk area could be justified. I could see that happening to a degree as the models often underforecast the northern extent of the warm sector in dynamically-strong cases such as this one, but I don’t think it will be as significant as the SPC appears to think it will be.

Some parts of the Mid-Atlantic have received and are still getting anything from light flurries up to a half inch of snow from bay-enhanced and ocean-enhanced snow this morning:

Steep lapse rates right near the surface and a veering wind profile are allowing the low-level clouds to suck up moisture from the Atlantic Ocean and from the Chesapeake Bay, which gets deposited in the Mid-Atlantic as snow. The area of snowfall is expected to switch over to sleet and then rain or freezing rain eventually as the mid-level temperatures warm. It is not a big concern as the precipitation remains light and widely scattered in nature, but it is something to take note of in areas that see more persistent precipitation.